We're back. Sorry for the missing issues. I know this journal has a small, but extremely devoted following that needs its monthly dosage of new tube circuits. I would like to blame the Macarthur Grant selection committee, but then they probably believe that this journal makes money. Actually, time was against me. I have spent the last three months working on a tube manual program that will go on sale soon. The program uses rich text formatting, filters data effectively and displays and prints tube curves. It is a big program, but the topic is even bigger. The tubes… there are thousands and thousands of types. And there are headaches everywhere: the same tube with different names; different tubes with the same name; pentode routinely referred to as tetrodes; conflicting specifications from one manual to another; and informational blank spots that cannot be filled.
    In this issue, output transformer testing is covered nicely by John Atwood and I tackle how to make tube amplifiers safer.
    Remember, if you have a request or suggestion of your own for either an article topic or circuit explanation, please e-mail:         
                                 Editor

    Having tested a lot of audio transformers, I thought that I had the procedure down. But when a friend's transformer high frequency response measurements were significantly different than mine (for the same transformers), I decided to get to the bottom of the differences.
    The transformers in question were single-ended ones meant for shunt-feed, so the discussion will focus on this type, although the conclusions also apply to conventional single-ended and push-pull output transformers.


The Set-up
      The ultimate test of a transformer is how it performs in the actual circuit it is intended for. However, when designing and testing a transformer for use by many people in many circuits, standard tests that can be done on the test bench with audio test equipment are needed. I have been using a test set-up as shown in figure 1. Over the years the sine-wave oscillator and meter used have gone from Heathkits to Hewlett-Packards to a Sound Technology 1700B, and now an Audio Precision System 2. For output transformers intended to be driven by a triode, Rs is set to 1/2 the rated transformer output impedance and RL is set to the rated load impedance. This is the set-up recommended by Reuben Lee in his Electronic Transformers and Circuits, 2nd ed., fig. 113.